1. Field of the Invention
The present invention relates to liquid crystal display devices and, more particularly, to a liquid crystal display device including a flexible circuit which is provided along one of short sides of a liquid crystal display element formed by sandwiching a liquid crystal layer with a first substrate having active elements formed thereon and a second substrate having a common electrode formed thereon and which is connected at one end thereof to an interface circuit board to supply a driving signal voltage for display to said active elements and a driving IC chip which is mounted such that it connected to extraction lines of the active elements at output terminals thereof and to a conductive layer portion of said flexible board at input terminals thereof.
2. Description of the Related Art
Liquid crystal display devices arc widely used as display devices capable of high definition color display for use in notebook type computers and display monitors.
Known liquid crystal display devices include liquid crystal display devices of the passive matrix type utilizing a liquid crystal panel sandwiched by a pair of substrates formed with parallel electrodes which are formed on the respective inner surfaces so as to cross each other and of the active matrix type utilizing a liquid crystal display element having switching elements for selection in pixels on one of a pair of substrates (hereinafter also referred to as liquid crystal panel).
Active matrix type liquid crystal display devices include the so-called vertical field type liquid crystal display devices, represented by the twisted nematic (TN) system, utilizing a liquid crystal panel having a group of electrodes for pixel selection formed on each of a pair of upper and lower substrates (generally referred to as TN system active matrix type liquid crystal display devices) and the so-called horizontal field type liquid crystal display devices utilizing a liquid crystal panel having a group of electrodes for pixel selection formed on only one of a pair of upper and lower substrates (generally referred to as IPS system liquid crystal display device).
In a liquid crystal panel forming a part of the former TN system active matrix type liquid crystal display device, liquid crystal is oriented at al twist of 90xc2x0 within a pair of substrates (two sheets consisting of a first substrate (lower substrate) and a second substrate (upper substrate)), and two polarizing plates are formed on outer surfaces of the upper and lower substrates of the liquid crystal panel which are in the crossed Nicols configuration in the directions of absorption axes thereof and whose absorption axis at the entrance side is in parallel with or orthogonal to the rubbing direction.
In such a TN system active matrix type liquid crystal display device, incident light is tuned into linearly polarized light by the entrance side polarizing plate when no voltage is applied; the linearly polarized light propagates along the twist of the liquid crystal layer; and, when the transmission axis of the exit side polarizing plate coincides with the azimuth angle of said linearly polarized light, the linearly polarized light is entirely emitted to show white display (the so-called normally open mode).
When a voltage is applied, the direction of a unit vector (director) indicating an average direction of orientation of the molecular axes of the liquid crystal forming the liquid crystal layer is in a direction perpendicular to the plane of the substrates. The azimuth angle of the linearly polarized light at the entrance side is thus unchanged and therefore coincides with the absorption axis of the exit side polarizing plate, which results in black display (see Principles and Applications of Liquid Crystal issued by Institute for Industrial Research, 1991).
In an IPS system liquid crystal display in which a group of electrodes for pixel selection and a group of electrode wirings are formed only on one of a pair of substrates and in which a voltage is applied between adjoining electrodes on said substrate (between a pixel electrode and a counter electrode) to switch the liquid crystal layer in a direction parallel with the plane of the substrates, polarizing plates are provided to show black display when no voltage is applied (the so-called normally closed mode).
The liquid crystal layer of an IPS system liquid crystal display device is in a homogeneous orientation in parallel with the plane of the substrates in the initial state. The director of the liquid crystal layer is in parallel with or at some angle to the direction of the electrode wiring in a plane parallel with the substrates when no voltage is applied. During the application of a voltage, the direction of the director of the liquid crystal layer moves to the direction perpendicular to the direction of the electrode wiring as the voltage is applied. When the direction of the director of the liquid crystal layer is inclined toward the direction of the electrode wiring at 45xc2x0 from the direction of the director under no application voltage, the liquid crystal layer under the application voltage causes the azimuth angle of the polarized light to rotate at 90xc2x0 as if it is a xc2xd wavelength plate. As a result, the transmission axis of the exit side polarizing plate and the azimuth angle of the polarized light coincide with each other to show white display.
The IPS system liquid crystal display device is characterized in that it has less variation of hues and contrast associated with the field angle to allow a wider field angle (see Japanese unexamined patent publication No. H5-505247).
Color filter systems are the main stream of attempts to make full-color versions of the various liquid crystal display devices described above. This is achieved by separating a pixel corresponding to one dot in a color display into three parts and by providing color filters corresponding to three respective primary colors, e.g., red (R), green (G) and blue (B), at respective unit pixels.
While the present invention can be applied to the various liquid crystal display devices described above, it will now be briefly described with reference to a TN system active matrix type liquid crystal display device as an example.
As described above, in a liquid crystal display element (liquid crystal panel) forming apart of a TN system active matrix type liquid crystal display device (hereinafter simply referred to as active matrix type liquid crystal display device for simplicity), there is formed a group of gate lines which extend in an x-direction in parallel with a y-direction and a group of drain lines which are insulated from the group of gate lines and which extend in the y-direction in parallel with the x-direction, on the surface toward a liquid crystal layer, of one of two transparent insulated substrates made of glass or the like provided in a face-to-face relationship with each other with the liquid crystal layer interposed therebetween.
Each of regions surrounded by the group of gate lines and the group of drain lines serves as a pixel region and, for example, a thin film transistor (TFT) and a transparent pixel electrode are formed in such a pixel region as active elements (switching elements).
When a scan signal is supplied to a gate line, the thin film transistor is turned on, and an image signal from the drain line is supplied to the pixel electrode through the thin film transistor which has been turned on.
Each gate line among the group of gate lines, not to mention each drain line among the group of drain lines, is also extended to the periphery of the substrate to form an external terminal, and an image driving circuit and a gate scan driving circuit connected to each of the external terminals, i.e., a plurality of driving IC chips (semiconductor integrated circuits which are hereinafter also, simply referred to as driving ICs or ICs) forming the sane arc externally mounted on the periphery of the substrate. That is, a plurality of tape carrier packages (TCPs) carrying those driving ICs are externally mounted on the periphery of the substrate.
However, since such a substrate is configured to allow TCPs carrying driving ICs to be externally mounted on the periphery thereof, the region between the contour of a display area formed by the regions where the group of gate lines and the group of drain lines cross on the substrate and the outer frame of the substrate (normally referred to as frame) occupies a large area, which goes against a need for reducing the external dimensions of a liquid crystal display module obtained by integrating a liquid crystal display element with an illuminating light source (back light) and other optical elements.
Therefore, in order to mitigate such a problem as much as possible, i.e., in consideration to a need for mounting liquid crystal display elements with high density and for reducing the external dimensions of liquid crystal display modules, the so-called flip-chip system or chip-on-glass (COG) system has been proposed in which image driving ICs and scan driving ICs are loaded directly on one of substrates (lower substrate) instead of using TCP components.
Japanese patent application No. H6-256426 by the same applicant relates to a liquid crystal display device based on such a flip-chip system.
For example, a liquid crystal display device of this type is formed by laminating a first substrate having on its periphery driving ICs for supplying driving signals to switching elements such as thin film transistors for driving pixels formed at intersections between a group of gate lines and a group of drain lines and a second substrate formed with a common electrode in a face-to-face relationship with each other, providing a liquid crystal display device by sandwiching a liquid crystal layer in the gap of the lamination, overlaying an illuminating light source for illuminating the liquid crystal display element, and securing them with an upper case and a lower case.
FIG. 43 is a schematic plan view of major parts for explaining the arrangement of a drain-driving IC for a liquid crystal display element and a drain-side flexible board forming a part of a conventional liquid crystal display device. SUB1 represents a first substrate (lower substrate) on which thin film transistors are formed; SUB2 represents a second substrate (upper substrate) on which color filters are formed; DTM represents drain extraction lines formed on the lower substrate; Td represents terminal wiring for connecting a protrusion (conductive layer portion) FSL of a drain-side flexible board FPC2 and a driving IC; COM represents a common electrode wiring for supplying power to a common electrode on the upper substrate SUB2; COMT represents a common electrode wiring terminal portion for electrically connecting the common electrode wiring COM to said common electrode through conductive paste or the like; ALC represents an alignment mark on the lower substrate SUB1; FHL represents a locating hole of the drain-side flexible board FPC2; AR represents a display area on a liquid crystal display panel; ARR represents a group of drain lines connected to one driving IC; CHD represents a chip capacitor for reducing noises on a power supply line of the flexible board FPC2; and a, b and c represent lines indicating the center of ARR, the center of the driving IC and the center of FSL, respectively.
Since the common electrode wiring COM for supplying power to the common electrode on the upper substrate SUB2 and the alignment mark ALC for defining the position of the liquid crystal display element during assembly are provided on the corner of the lower substrate which is furthest from an interface circuit board PCB of the liquid crystal display element, the driving IC and the conductor portion FSL, i.e., protrusion, of the flexible board FPC2 located in this region have been offset toward the interface circuit board PCB.
Specifically, as shown in FIG. 43, the center b of the driving IC in the direction parallel with the direction in which the extraction lines DTMP of active elements are arranged has been offset from the center a of the active elements driven by the driving IC in the direction in which the extraction lines DTM are arranged toward the interface board PCB (xcex81 less than xcex82), and the center c of the protrusion FSL of the flexible board connected to the input terminals Td of the driving IC chip in the direction parallel with the direction in which the extraction lines DTM are arranged has been matched with the center b of the driving IC chip (xcex83=xcex84).
The wiring density of the extraction lines DTM of the active elements is higher than that of the input terminals Td, and the angle xcex81 of the diagonal wiring thereof is limited when consideration is paid to the difference in the wiring resistance between one end of the group of drain lines ARR connected to one driving IC and another end thereof.
In the above-described conventional configuration the angle xcex81 of the diagonal wiring of the extraction lines DTM closer to the corner of the lower substrate SUB1 is considerably acute, and such a diagonal wiring portion DTMS must be made small when the difference in wiring resistance is considered, which results in an increase in the length of a normal wiring portion DTMP.
This leads to an increase in the width D of the mounting area of the driving IC on the lower substrate SUB1, which has resulted in a problem in that it hinders efforts toward so-called smaller frames to discourage the promotion of smaller frames.
It is an object of the present invention to solve the above-described problem and to improve the arrangement of extraction lines DTM of drain wiring, a driving IC for the same and a drain-side flexible board FPC2, thereby providing a liquid crystal display device in which the dimensions of the frame area can be reduced.
In order to achieve the above-described object, according to the present invention, a driving IC is mounted such that extraction lines DTM of drain wiring are inclined at angles to achieve symmetry about the center thereof (xcex81=xcex82) and the angles of inclination of terminal wiring Td for connecting a protrusion (conductive layer portion) FSL of a flexible board FPC2 having a relatively low wiring density and the driving IC at both ends thereof are different from each other (xcex83 greater than xcex84). That is, the present invention is characterized in that it has a configuration as described below.
(1) There is provided a liquid crystal display element formed by sandwiching a liquid crystal layer with a first substrate having active elements formed thereon and a second substrate having a common electrode formed thereon, an interface circuit board provided along one of short sides of said liquid crystal display element, a flexible board connected to said interface circuit board at one end thereof and provided at least along one long side of said first substrate for supplying a driving signal voltage for display to said active elements and a driving IC chip mounted such that it is connected to extraction lines of said active elements at output terminals thereof and to a conductor layer portion of said flexible board at input terminals thereof.
The center of the extraction lines of said active elements driven by said one driving IC chip in the direction of the arrangement thereof is matched with the center of said driving IC chip in the direction parallel with the direction of the arrangement of the extraction lines of said active elements, and the center of the conductive layer portion of said flexible board connected to the input terminals of said one driving IC chip in the direction parallel with the direction of the arrangement of the extraction lines of said active elements is offset from the center of said driving IC chip toward said interface circuit board.
In this configuration, the angle of inclination of the extraction lines DTM of the drain wiring having a high wiring density is moderated as a whole to eliminate the need for increasing the normal wiring portion DTMP and to thereby promote the trend towards smaller frame areas.
(2) Said one driving IC chip per (1) is mounted on said flexible board in the position furthest from said interface circuit board.
This configuration satisfies the need for offsetting the conductive layer portion of the flexible board toward the interface circuit board PCB in the region where the common electrode wiring COM and the lie are provided to increase the flexibility in designing the pattern of the lower substrate SUB1 in addition to the effect as mentioned in the above (1 1).
The present invention is not limited to active matrix type liquid crystal display devices as described above and may be applied similarly to liquid crystal display devices of other types or other electronic apparatuses which require similar conductor patterns.